1. Field of the Invention
The present invention relates to photothermographic materials. In particular, the invention pertains to a photothermographic material which is water-base coated.
2. Description of the Related Art
In recent years, reduction of a quantity of a waste processing solution is strongly desired in the field of medical diagnosis from the standpoints of preservation of the environment and space saving. Therefore, a technique relating to a photothermographic material, for use in the medical diagnosis and a photographic art, which is capable of being efficiently exposed with a laser image setter or a laser imager and forming a clear black image having high resolution and sharpness, is required. The photothermographic material can provide users with a simple and non-polluting thermal development processing system that eliminates the use of solution type processing chemicals.
It is characteristic of an image for use in the medical diagnosis that, a high image quality excellent in sharpness and graininess is necessary due to a requirement for a fine depiction thereon and thus, a cold monochromatic image is preferred from the viewpoint of the ease of diagnosis. At present, various types of hard copy systems utilizing a pigment or a dye, such as an ink jet printer and an electronic photographic system, are distributed as an general image forming system. However, none of the hard copy systems is satisfactory as an output system of the image for use in the medical use.
Meanwhile, photothermographic systems utilizing an organic silver salt are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and B. Shelly, “Thermally Prossessed Silver Systems” (Imaging Prossesses and Materials, Neblette, 8th Ed. compiled by J. Sturge, V. Walworth and A. Shepp, p. 2, 1996).
In particular, the photothermographic material comprises a photosensitive layer in which a catalytically active quantity of photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and, optionally, a color toning agent for controlling color tone of silver are ordinarily dispersed in a binder matrix. When the photothermographic material is heated at a high temperature (for example, 80° C. or more) after being exposed imagewise, a monochromatic black silver image is produced by an redox reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic action of a latent image of the silver halide generated by such exposure. Therefore, the black silver images are formed in exposed areas of the material. This technique is disclosed in many references including U.S. Pat. No. 2,910,377 and JP-B No. 43-4924. Then, Fuji Medical Dry Imager FM-DP L was put into the market as an image forming system for medical diagnosis utilizing the photothermographic material.
In manufacturing of the photothermographic system utilizing the organic silver salt, the system has been produced so far by a coating with an organic solvent. Examples of such organic solvents include methyl ethyl ketone (MEK), toluene, and acetone. When the solvents were coated and dried, a post-treatment, such as recovering or drying, was necessary for preventing diffusion of the solvents into air, which caused enlargement of production facilities and increase in production cost. Further, there was a problem that the solvents remained in products and, when the products were handled, particularly at a thermal development, the solvents evaporated from a film thereof to emit a odor, or diffused into the environment to pollute it. In recent years, a water-base coating type photothermographic material which is produced by coating and drying a coating solution that contains various types of components in a dispersed state and polymer fine particles as a binder in a form of an aqueous dispersion, has been developed. Then, Fuji Medical Dry Imager FM-DP L was put into the market as an image forming system for medical diagnosis utilizing such system as described above. Since the latter system is capable of using high-speed type facilities for coating and drying of the emulsion common to production of a conventional wet-type photosensitive material and performing production by a high-speed wide coating, it is advantageous in mass-production.
The organic silver salt has ordinarily been dispersed in an organic solvent and used so far. However, in case of the water-base coating type photothermographic material. it is necessary to disperse finely the organic silver salt in water. Particularly, in order to exhibit transparency of a coated film, a uniformly coated surface state, and a stable photographic performance, it is necessary to use the organic silver salt dispersed finely and uniformly. For such dispersion, a media dispersion (ball mill, sand grinder mill or the like) using balls, such as those of iron, zirconia, and glass, as dispersion media has ordinarily been utilized.
However, when the organic silver salt is dispersed in water, it has been found that shelf-stability, fogging and the like of the photothermographic material are substantially affected by condition of such dispersion. In other words, as the dispersion becomes finer, uniformity of a coated film of the photothermographic material produced by coating such dispersion is improved and speeding-up of development can also be aimed, but an adverse effect has been found at the same time that, the fogging is increased and the shelf-stability is deteriorated.
It is conventionally known that a solid fine particle dispersion of an organic compound is subjected to an annealing treatment. It is described in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 5-216166, 5-313307 and 8-201975 that a spectral absorption spectrum of a dye is adjusted or dispersion stability thereof is improved by subjecting a dye solid dispersion to the annealing treatment. Further, it is described in, for example, JP-A No. 2002-55405 that a dispersion of a bisphenol compound, which is a reducing agent used in the photothermographic material, a polyhalogen compound as an antifoggant, and a compound capable of forming a hydrogen bond with the bisphenol compound is subjected to the annealing treatment for trying to improve a sedimentation stability of the dispersion, or stability of the dispersion against agglomeration thereof during storage.
However, it can not be anticipated from these descriptions that the annealing treatment of the organic silver salt is connected with solution of the problem recited in the present invention.